Free DevOps tool
AWS VPC Subnet Planner
Generic subnet calculators do one subnet at a time. A VPC is a plan: enter your VPC CIDR, pick a subnet size and how many you need, and this planner carves the block up across your availability zones — showing every subnet's CIDR, network, first and last usable address, broadcast, and the AWS-usable host count (total minus the 5 addresses AWS reserves), plus the free space left over for the tiers you have not built yet. No sign-up; nothing leaves your browser.
/24 = 251 usable · /20 = 4,091 · /28 = 11
0 = don't label subnets by AZ
Everything runs in your browser. AWS reserves 5 addresses in every subnet (network, VPC router, DNS, future use, broadcast), so usable hosts are always the subnet size minus 5.
VPC
10.0.0.0/16
65,536 addresses
Allocated
6 × /24
1,536 addresses
Left free
64,000
addresses for future tiers
Your subnets
| # | AZ | CIDR | Network | First usable | Last usable | Broadcast | Addresses | Usable (AWS) |
|---|---|---|---|---|---|---|---|---|
| 1 | AZ a | 10.0.0.0/24 | 10.0.0.0 | 10.0.0.4 | 10.0.0.254 | 10.0.0.255 | 256 | 251 |
| 2 | AZ b | 10.0.1.0/24 | 10.0.1.0 | 10.0.1.4 | 10.0.1.254 | 10.0.1.255 | 256 | 251 |
| 3 | AZ c | 10.0.2.0/24 | 10.0.2.0 | 10.0.2.4 | 10.0.2.254 | 10.0.2.255 | 256 | 251 |
| 4 | AZ a | 10.0.3.0/24 | 10.0.3.0 | 10.0.3.4 | 10.0.3.254 | 10.0.3.255 | 256 | 251 |
| 5 | AZ b | 10.0.4.0/24 | 10.0.4.0 | 10.0.4.4 | 10.0.4.254 | 10.0.4.255 | 256 | 251 |
| 6 | AZ c | 10.0.5.0/24 | 10.0.5.0 | 10.0.5.4 | 10.0.5.254 | 10.0.5.255 | 256 | 251 |
“First usable” starts at network + 4 because AWS reserves .0 (network), .1 (VPC router), .2 (DNS) and .3 (future use) on top of the broadcast address.
Remaining free space in 10.0.0.0/16
What generic subnet calculators get wrong about AWS
The minus-5 is real capacity
A textbook /24 has 254 usable hosts. In AWS it has 251 — the network, router, DNS and future-use addresses at the start, plus broadcast at the end, are never yours. On a /28 the tax is brutal: 16 addresses, 11 usable. Planning with textbook numbers is how a subnet fills up one instance early.
Leftover space is a design decision
A VPC CIDR can never shrink or move, and subnets are immutable once created. The free blocks this planner lists are your future: the data tier you add next quarter, the EKS cluster that needs its own ranges. Consume the whole block on day one and the only way out is a new VPC.
Subnets are per-AZ, always
A subnet lives in exactly one availability zone. High availability means the same tier repeated per AZ — which multiplies the subnet count and is why the planner labels subnets by AZ instead of leaving you to track it in a spreadsheet.
Bigger VPC, smaller regrets
RFC 1918 space is free. A /16 costs nothing more than a /20 and removes CIDR exhaustion from your future entirely — the standard advice is to take the /16 and carve disciplined subnets out of it, which is the workflow this tool is built around.
Related guides
AWS VPC Guide 2026: Design a Network That Never Boxes You In
The full VPC design walkthrough: choosing a CIDR, public/private/data tiers across AZs, route tables, NAT economics and flow logs.
Kubernetes Ingress & Networking Basics (2026)
How cluster networking builds on the VPC underneath it — Services, Ingress, and where the subnet layout shows up in EKS.
Cron Translator
One cron schedule converted for crontab, Kubernetes CronJob, GitHub Actions and AWS EventBridge — with next run times in UTC and JST.
Frequently asked questions
How many usable IPs does an AWS /24 subnet have?
251. A /24 holds 256 addresses, and AWS reserves 5 in every subnet: the network address (.0), the VPC router (.1), the DNS server (.2), one reserved for future use (.3), and the broadcast address (.255). 256 − 5 = 251 usable for instances, containers and load balancer nodes.
What subnet sizes does AWS allow in a VPC?
Both the VPC CIDR and each subnet must be between /16 and /28. A /16 VPC holds 65,536 addresses; a /28 subnet is the smallest AWS allows, with 16 addresses and 11 usable. Subnets must be equal to or smaller than the VPC block (a prefix number equal to or larger than the VPC prefix).
How many subnets do I need per VPC?
A common production layout is three tiers — public, private and isolated/data — across two or three availability zones, so 6 or 9 subnets. This planner labels subnets round-robin across your AZ count; assign tiers by taking consecutive groups (for example, with 9 subnets across 3 AZs, plan one tier at a time or split the VPC into per-tier blocks first).
Can I resize a VPC or a subnet later?
No. A VPC CIDR can gain additional (secondary) CIDR blocks but the primary block never changes, and a subnet's CIDR is fixed at creation. That permanence is exactly why planning the split before creating anything — including leaving free space for future tiers — is worth two minutes in a tool like this one.
Why does the first usable address start at .4?
Because AWS reserves the first four addresses of every subnet: .0 is the network address, .1 is the VPC router, .2 is the DNS server (AmazonProvidedDNS), and .3 is reserved for future use. Combined with the broadcast address at the end, that is the "minus 5" in every AWS usable-host count.